1. Field of the Invention
The present invention relates to a white balance correction circuit of a color image receiving tube in which the gamma characteristics of three electron guns provided within the color image receiving tube which emit red (R), green (G) and blue (B) video signals, are maintained in agreement to thereby maintain white balance.
2. Description of Related Art
Generally, white balance refers to reproducing achromatic images by an additive matrix process with respect to red (R), green (G) and blue (B) colors. In the case of the color image receiving tube having three electron guns, there needs to be control of the operation of R, G and B electron guns or the magnitude of respective video signals due to variations in the luminous efficiency of the phosphors associated with the scanning of each electron gun. Incomplete control can not bring about high quality colored images for reproduction. Therefore, in order to reproduce beautifully colored images, there is a need to correct a color difference signal or a color signal, achieving the picture quality correction by noise removal, contour compensation, black-level correction, gamma correction (which is referred to as intermediate luminance correction), etc.
FIG. 1 shows a curve representing the gamma characteristics of the R, G and B electron guns in a color image receiving tube. In FIG. 1, a horizontal axis depicts to a voltage applied to the control grid of the color image receiving tube, while the vertical axis shows luminous outputs in terms of percentage. When a color signal of a specified voltage is supplied to the color image receiving tube, a luminous quantity is changed according to the luminous characteristics of the color image receiving tube. Such an electro-photic conversion characteristic is referred to as the gamma characteristic, which signifies luminous quantity when an electric signal supplied to electron guns produces an emission as a respective electronic beam. Referring to FIG. 1, in the case that luminous quantities are respectively a first level L1 of 25% and a second level L2 of 75%, luminous efficiency with respect to the three color signals agrees, thereby permitting an acceptable white balance. However, when the voltage level is between the first level L1 and second level L2, luminous efficiency for the green color signal G is higher than that of the red and blue color signals R and B. Accordingly, it is difficult to reproduce a clear colored image because the white balance is now inclined toward the green side.
A delta type of color image receiving tube which has a triangular arrangement of electron guns, is formed so as to permit control of the voltages supplied to the control grid and screen grid for each electron gun. When the electron guns show luminous quantities similar to those of FIG. 1, the white balance can be adjusted by the control of the voltages provided to the control grid and screen grid.
In contrast, an in-line type of color image receiving tube, which has three electron guns arranged in series employs a self-convergence method for maintaining white balance. The format wherein the control grid and screen grid are united with electron guns provides a simple construction and permits a reduction of voltage consumption owing to low deflection voltages. However, since the in-line type of the color image receiving tube can not control the voltages of the control grid and screen grid associated with the electron guns, this configuration has a drawback that the white balance of the video signal is often broken.
Japanese Patent No. 4-298189 was published on Oct. 21, 1992 and discloses a prior art device for stabilizing the white balance under control of the gamma characteristics of the three colors. The prior art discloses a technology whereby the gamma characteristics with respect to the three primary colors are maintained in agreement using a digital-gamma correction circuit having a ROM table.